Field
This disclosure relates to data encoding and decoding.
Description of Related Art
The “background” description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description which may not otherwise qualify as prior art at the time of filing, is neither expressly or impliedly admitted as prior art against the present disclosure.
There are several video data encoding and decoding systems which involve transforming video data into a frequency domain representation, quantising the frequency domain coefficients and then applying some form of entropy encoding to the quantised coefficients. This can achieve compression of the video data. A corresponding decoding or decompression technique is applied to recover a reconstructed version of the original video data.
Current video codecs (coder-decoders) such as those used in H.264/MPEG-4 Advanced Video Coding (AVC) achieve data compression primarily by only encoding the differences between successive video frames. These codecs use a regular array of so-called macroblocks, each of which is used as a region of comparison with a corresponding macroblock in a previous video frame, and the image region within the macroblock is then encoded according to the degree of motion found between the corresponding current and previous macroblocks in the video sequence, or between neighbouring macroblocks within a single frame of the video sequence.
High Efficiency Video Coding (HEVC), also known as H.265 or MPEG-H Part 2, is a proposed successor to H.264/MPEG-4 AVC. It is intended for HEVC to improve video quality and double the data compression ratio compared to H.264, and for it to be scalable from 128×96 to 7680×4320 pixels resolution, roughly equivalent to bit rates ranging from 128 kbit/s to 800 Mbit/s.
In HEVC a so-called 4:2:0 block structure is proposed for consumer equipment, in which the amount of data used in each chroma channel is one quarter that in the luma channel. This is because subjectively people are more sensitive to brightness variations than to colour variations, and so it is possible to use greater compression and/or less information in the colour channels without a subjective loss of quality.
HEVC replaces the macroblocks found in existing H.264 and MPEG standards with a more flexible scheme based upon coding units (CUs), which are variable size structures.
Consequently, when encoding the image data in video frames, the CU sizes can be selected responsive to the apparent image complexity or detected motion levels, instead of using uniformly distributed macroblocks. Consequently far greater compression can be achieved in regions with little motion between frames and with little variation within a frame, whilst better image quality can be preserved in areas of high inter-frame motion or image complexity.
Each CU contains one or more variable-block-sized prediction units (PUs) of either intra-picture or inter-picture prediction type, and one or more transform units (TUs) which contain coefficients for spatial block transform and quantisation.
Moreover, PU and TU blocks are provided for each of three channels; luma (Y), being a luminance or brightness channel, and which may be thought of as a greyscale channel, and two colour difference or chrominance (chroma) channels; Cb and Cr. These channels provide the colour for the greyscale image of the luma channel. The terms Y, luminance and luma are used interchangeably in this description, and similarly the terms Cb and Cr, chrominance and chroma, are used interchangeably as appropriate, noting that chrominance or chroma can be used generically for “one or both of Cr and Cb”, whereas when a specific chrominance channel is being discussed it will be identified by the term Cb or Cr.
Generally PUs are considered to be channel independent, except that a PU has a luma part and a chroma part. Generally, this means that the samples forming part of the PU for each channel represent the same region of the image, so that there is a fixed relationship between the PUs between the three channels. For example, for 4:2:0 video, an 8×8 PU for Luma always has a corresponding 4×4 PU for chroma, with the chroma parts of the PU representing the same area as the luma part, but containing a smaller number of pixels because of the subsampled nature of the 4:2:0 chroma data compared to the luma data in 4:2:0 video. (In 4:4:4 the sampling rates are the same and so the same PU structures can be used for luma and chroma samples). The two chroma channels share intra-prediction information; and the three channels share inter-prediction information. Similarly, the TU structure also has a fixed relationship between the three channels.
However, for professional broadcast and digital cinema equipment, it is desirable to have less compression (or more information) in the chroma channels, and this may affect how current and proposed HEVC processing operates.